Steel mill waste effects on rice growth: comparison of chemical extractants on lead and zinc availability

Zinc deficiency is widespread in cultivated soils, limiting the grain crop production and the adequate human nutrition. Several wastes from metallurgical activity can be used as Zn source, but these materials generally also have other potentially toxic elements, such as Pb, that can be highly toxic for plants and humans. This study aimed to evaluate the efficiency of five chemical extractors (water, citric acid, DTPA, Mehlich 1, and USEPA 3051A) in better correlating with the bioavailable contents of Zn and Pb in soils treated with steel mill wastes (metallurgic press residue (MPR), filter press mud (FPM), and phosphate mud (PM)). Rice plants were cultivated in pots with 4 kg of a Haplic Eutrophic Gleisol and steel mill wastes were applied in soil at increasing doses (0, 0.5, 1, 2, 4, 8, and 16 t ha⁻¹). The availability of the potentially toxic elements Zn and Pb was assessed as total contents in rice shoots, grains, husks, and roots. The results showed that the USEPA 3051A method extracted greater contents of Zn and Pb from soil compared with other extractants. Due to their greater natural Pb and Zn contents, MPR and PM promoted higher contents of these elements in soils, respectively. Doses of PM influenced Zn contents in grains. After adding 16 t ha⁻¹ of PM, Zn content in rice grains was 0.1 mg kg⁻¹. However, at doses 0.5, 1, 2, and 4 t ha⁻¹, the average concentration of Zn in the grains was 40 mg kg⁻¹. The wastes MPR and FPM at 16 t ha⁻¹ promoted Zn concentration in grains of 42 and 45 mg kg⁻¹, respectively. The greatest contents of Pb in grains were found after addition of FPM at doses 0.5, 1, and 2 t ha⁻¹: 6.67, 4.96, and 0.45 mg kg⁻¹, respectively, and above 4 t ha⁻¹ (4, 8, and 16 t ha⁻¹); Pb content in grains was less than 0.3 mg kg⁻¹. The content of Pb in roots at 16 t ha⁻¹ of PM, MPR, and FPM was 18, 25, and 155 mg kg⁻¹, respectively, and for Zn, under the same conditions, 100, 255, and 813 mg kg⁻¹ for MPR, FPM, and PM, respectively. USEPA 3051A can be used to assess Pb and Zn available contents, and positive correlations with bioavailable contents of these elements in roots prove its feasibility. Further studies are necessary to state the safety of using steel mill application, including the use of other crop species, but PM is a promising waste for soil Zn fertilization.